Try and see if you can isolate the mikes, by placing a screen in front of the mikes between them and the speakers- what happens with feeed back is the sound from the mike gets amplified and then is picked up again by the mikes and run around again and again, the high frequency gets doubled and doubled till you get feedback. I know of a guy who does home recording and he made a baricade in a corner of the room with a matress as a partition...the result, no, room ring, no feedback and nice clean damped vocals!
Speaker position is important also, as are quality- I have a pair od Mackie srm 450s and they have an inbuilt circuit that time delays the treble by millisecs and this cuts feedback a lot! Another way around is not the best as you can loose the feel of the song is add the vocals later!
Let me know how you solve this- I hope this helps a little for ideas mate-Good luck
H